1. Field of the Invention
The present invention is related to microwave data transmission on optical/infrared carrier signals by means of optical waveguides and, more particularly, is directed toward a means for effectively transferring and modulating a coherent source of optical/infrared radiation from its source path to a transmission line output device.
2. Description of the Prior Art
Optical waveguides of both the glass and single crystal types have been under intensive development for the purposes of data transmission. Optical waveguides are considerably more attractive for certain applications when compared with conventional transmission lines, such as coaxial cable and the like. For one thing, optical waveguides exhibit extremely low loss characteristics and are available in substantial lengths. For example, losses of less than 2 db/km in glass waveguides and lengths in excess of 2 km have been reported. Furthermore, optical waveguides are not subject to interference from electromagnetic radiation. Additionally, power handling capabilities in excess of 0.8 mw. per square cm. have been reported. Data bandwidth in excess of 10 GHz. can be transmitted over distances of up to 1 kilometer without serious distortion, and amplitude and heterodyne detection can process comparable data frequencies.
There do, however, exist serious problems in the areas of broadband modulation and insertion of coherent radiation into optical waveguides. State-of-the-art systems typically utilize laser diodes with optics to focus part of the optical signal into the end of a glass waveguide. Data modulation is achieved typically by modulating the voltage to the laser diode. This technique is, however, limited to relatively low power and is quite inefficient. Other prior art techniques utilize gas, crystal, or glass lasers, electro-optics or acousto-optic modulators and associated complex techniques for getting or transmitting the modulated carrier into the optical fiber waveguides. All of such techniques have exhibited problems in the areas of efficiency, power level achievable, and data bandwidth, to name a few.
Accordingly, while the development of glass and single crystal waveguides of the type which transfer gigahertz bandwidth data over optical or infrared carriers has been rapid, and while detection of such data at the far end of such optical waveguides is presently accomplished rather efficiently, there exist serious problems in the front end as concerns broadband modulation and insertion of coherent radiation into the waveguides, and clearly a more efficient technique and arrangement are needed.
One particular application of such optical information transmission systems which has both made me aware of prior art problems and has given rise to the present inventive technique concerns an indoor radar testing facility which utilizes an array of RF modules positioned on a nylon cable grid. Fiber optic waveguides are utilized to carry the radar signals to the array, with coherent optical radiation being utilized as the carrier signal. As pointed out above, presently available techniques do not lend themselves to efficient coupling of the modulated carrier into the waveguide.